An important aspect of the manufacture of integrated circuit chips is the testing of the circuit embodied in the chip in order to verify that it operates according to specifications. Although the circuit could be tested after the chip has been packaged, the expense involved in dicing the wafer and packaging the individual chips makes it desirable to test the integrated circuit as early as possible in the fabrication process, so that unnecessary efforts will not be expended on faulty devices. It is therefore desirable that these circuits be tested either immediately after wafer fabrication is completed, and before separation into dice, or after dicing but before packaging. In either case, it is necessary to make electrical connection to all the circuit's external connections (usually bonding pads) in a nondestructive way, so as not to interfere with subsequent packaging and connection operations.
It is desirable that an integrated circuit be tested under its design operating conditions and to the extremes of its design performance range. In particular, typical high speed circuits are designed to operate with input and output signal bandwidths exceeding 1 GHz, and it is necessary that operation of such circuits be evaluated at these high frequencies.
A high speed wafer probe is disclosed in U.S. Pat. Application Ser. No. 318,084 filed Nov. 4, 1981. A practical implementation of the probe disclosed in that patent application is capable of supporting signal bandwidths to approximately 18 GHz, but is able to provide only a few (less than ten) connections to the chip under test. Probes that are able to provide sufficient connections for the complex integrated circuits that are currently being manufactured, having fifty to one thousand bonding pads, have inadequate bandwidth for testing high speed circuits at the extremes of their performance range.
It has previously been proposed that an integrated circuit be tested using a probe comprising a body of elastomer having conductor runs of metals deposited on one face thereof. This probe is subject to a number of disadvantages. For example, the probe is not capable of supporting signals at frequencies above a few hundred megahertz without serious signal degradation, and it has poor mechanical stability owing to the large difference in elasticity of the elastomer body and the metallic conductor runs.
Co-pending application Ser. No. 812,145 filed Dec. 23, 1985, but now abandoned the disclosure of which is hereby incorporated by reference herein, discloses a probe assembly for use in testing an integrated circuit embodied in an integrated circuit chip. The probe assembly comprises a stiff support member formed with an aperture, and an elastically-deformable membrane. Both the support member and the membrane comprise dielectric material and portions of conductive material supported by the dielectric material in electrically-insulated relationship. The portions of conductive material of the membrane constitute inner contact elements distributed over a first main face of the membrane in a first pattern that corresponds to the pattern of contact areas on the contact face of the integrated circuit chip, outer contact elements distributed about a peripheral region of the membrane in a second pattern, and transmission lines extending from the inner contact elements to the outer contact elements respectively. The portions of conductive material of the support member comprise inner contact elements that are distributed about the aperture in a pattern corresponding generally to the second pattern, and transmission lines extending from the inner contact elements of the support member to testing apparatus. The membrane is secured to the support member so that it extends over the aperture, and the outer contact elements of the membrane are electrically connected to respective inner contact elements of the support member.
In a practical form of the probe assembly disclosed in co-pending application Ser. No. 06/812,145 , the support member is a circuit board that is disposed horizontally in use, and the inner contact elements of the support member are exposed at the upper surface of the support member. The outer contact elements of the membrane are exposed at the first main face of the membrane, and the membrane is clamped at its peripheral region to the upper surface of the support member using a body of elastomer material that spans the aperture in the support member. The first main face of the membrane is presented downwards, towards the interior of the aperture in the support member, and the chip is placed on a chip support that is sufficiently small to enter the aperture in the support member. The body of elastomer material has a downwardly-projecting protuberance that engages the membrane directly above the inner contact elements, so that when the pedestal is raised and the contact areas of the chip engage the inner contact elements of the membrane, upward deformation of the membrane is resisted in a resiliently yieldable fashion and the body of elastomer material supplies contact force for achieving pressure contact between the contact areas of the chip and the inner contact elements of the membrane. The maximum linear dimension of the aperture in the support member is smaller than the diameter of a standard semiconductor wafer. Because the support member is located below the membrane, and the inner contact elements are exposed to the chip under test through the aperture in the support member, the probe assembly is not well suited for testing integrated circuits in wafer form.
In the probe assembly disclosed in co-pending application Ser. No. 06/812,145 application, the transmission lines of the probe head are in a microstrip configuration, with the ground conductor on the opposite side of the membrane from the signal conductors, i.e. on the upper surface of the membrane. In a modification of the probe assembly, the transmission lines may be in microstrip configuration in a peripheral region of the film and in coplanar configuration (both the ground conductor and the signal conductors on the same side of the film) or grounded coplanar configuration (similar to coplanar configuration except that a ground plane is provided on the opposite side of the film from the signal conductors and is connected by plated through-holes to the ground conductor that is on the same side of the film as the signal conductors) closer towards the central region of the film. The transition is accomplished by providing vias through the film between the ground conductor on the lower surface of the film and the ground conductor on the upper surface of the film. Use of coplanar or grounded coplanar transmission lines has the advantage over the pure microstrip configuration of reducing cross-talk between the signal conductors and also possibly reducing losses because a greater part of field that is generated by a signal propagating along the transmission lines is in air rather than in the dielectric material of the film. However, formation of the vias requires an additional photoprocessing operation.